This application asserts the priority date of German Patent application No. 10104882.3, which was filed on Feb. 1, 2001.
The present invention concerns an activated carbon molded or shaped body, referred to hereinafter as a molded body.
The activated carbon molded body may be more particularly for example in honeycomb form and may be used as an adsorption filter.
The invention also concerns uses of the activated carbon molded body.
In the area relating to passenger compartment air filtration in motor vehicles, the requirements being made on odor filter systems are becoming increasingly demanding. The available structural space is becoming smaller and smaller and the filtration properties are to be better and better, with as far as possible a further reduction in the air resistance of the filter. A further problem is that the passive systems which are used nowadays can only be viewed as pure odor filter systems. Pollutants such as benzene and toluene, over the service life of the filter, become increasingly concentrated therein and are discharged again as from a certain level of loading in the filter. It is possible to avoid that undesirable side effect within the period of use of such a filter by increasing the amount of sorbents used. That inevitable results in an increase in the amount of structural space required or an increase in air resistance. Both of those are absolutely undesirable for new developments for motor vehicles and for the major part also cannot be implemented from a technical point of view in the present day air conditioning systems. Another alternative would be the premature replacement of a filter of that kind, which however is highly cost-intensive and in addition particularly time-intensive, as the replacement of such a filter generally has to be carried out in a specialist workshop.
A similar problem arises in the area of the tank venting systems for motor vehicles. The available structural space is becoming smaller and smaller but the demands on the filter systems are ever increasing for ecological reasons. Hitherto a reduction in the vapor fuels emitted from a tank has been achieved by the use of additional activated carbon. As there is only a limited structural space available it is not possible to travel too far along that path. That applies in particular if the admissible amount of fuel which passes into the environment within 24 hours has to be reduced to about 20% of the previously involved value, as is required for example in California as from the year 2001.
Many different endeavours have already been undertaken to satisfy the above-indicated requirements. In that respect, the men skilled in the art agree that these technical problems can only be satisfactorily resolved with adsorption filters of honeycomb form, as the honeycomb geometry affords the advantage of a large surface area for contact with a fluid flowing therethrough and only a low pressure drop at the filter.
Therefore, various operating procedures have been set forth, for producing activated carbon molded bodies of that kind involving a honeycomb structure. U.S. Pat. Nos. 4,399,052 and 5,820,967 each disclose honeycomb structures in which a starting material is extruded and then pyrolised. In that case the starting material is a mixture of various constituents, but activated carbon is not contained in the starting material. The honeycombs disclosed admittedly enjoy good abrasion resistance and hardness as well as suitable electrical conductivity for appropriate regeneration, but the mesoporous adsorber structure which is to be endeavoured to provide for tank ventilation cannot be produced with the resins disclosed for production purposes.
U.S. Pat. No. 5,515,063 proposes the production of an activated carbon molded body, without making use of an extrusion process. In this case, the passages in the activated carbon molded body are formed by a passage-forming material. Under pyrolysis conditions, the passage-forming material at least partially evaporates and leaves the passages behind in the honeycomb structure.
U.S. Pat. Nos. 6,097,011 and 4,518,704 disclose activated carbon molded bodies in which activated carbon is applied to a ceramic structure or matrix. In other words, the stability aspect is ensured by the ceramic main body, but the adsorption capability is limited because the total proportion of activated carbon in such activated carbon molded bodies is relatively low.
EP 0 492 081 discloses a honeycomb structure which is produced with the avoidance of a sintering operation at elevated temperatures. Therefore, production of the proposed honeycomb structure involves avoiding the use of clay or similar materials as the starting material, as relatively high temperatures would be required to produce a ceramic support structure. On the other hand, the use of relatively high temperatures for processing the main body of that structure is not possible as that results in cracks and fissures in the honeycomb structure.
Finally, U.S. Pat. No. 5,914,294 discloses a monolithic activated carbon molded body which, besides methyl cellulose, basically does not contain any organic constituents in the initial mixture employed. This has the disadvantage however or requiring relatively high combustion temperatures for producing the honeycomb structure.
An object of the present invention is to provide an activated carbon shaped or molded body which can be produced more particularly in honeycomb form and can be used as an adsorption filter.
Another object of the present invention is to provide an activated carbon body which is such that it can be produced by a relatively simple operating procedure.
Still another object of the present invention is to provide a use of an activated carbon molded body.
Yet another object of the present invention is to provide a process for the regeneration of an activated carbon molded body, which involves simple implementation with enhanced operational results.
In accordance with the present invention the foregoing and other objects are attained by an activated carbon molded body, more particularly for example in honeycomb form and for example for use as an adsorption filter, which can be obtained from a mixture containing activated carbon, water, novolak powder, clay, cellulose ether, liquid starch, wax, polyacrylamide and soap, by thoroughly mixing the constituents, extruding the mixture to form a monolithic shaped or molded body and cutting same to size, drying the molded body produced and pyrolysis thereof.
In the use aspect the foregoing and other objects are achieved by use of an activated carbon molded body as an adsorption filter.
Furthermore in the regeneration process the foregoing and other objects are attained by a process for the regeneration of an adsorption filter wherein the adsorption filter is subjected to the action of a heating power of between 10 and 35 W and more particularly between 10 and 23 W and a desorption air flow is set to between 0.1 and xe2x89xa70.65 m/sec.
The activated carbon molded body in accordance with the invention, after the pyrolysis operation, has an advantageously high activated carbon component of more than 70% by weight, which can afford persuasive efficiency data. Another particularly surprising consideration is the especially good electrical conductivity of the activated carbon body according to the invention, which makes it possible to implement regeneration of an activated carbon molded body which is used as an adsorption filter, by applying an electrical voltage thereto. The specific insulation resistance of the activated carbon molded body according to the invention, in relation to a cube with an edge length of 1 cm and measured between two mutually opposite side faces thereof, is between 0.1 and 5 xcexa9 at a temperature of 23xc2x0 C.
The mixture of the starting materials from which the item referred to as the green body is produced contains between 30 and 40% by weight of activated carbon, preferably between 32 and 38% by weight and particularly preferably between 34 and 37% by weight. The activated carbon is basically the main adsorption means in the activated carbon body according to the invention. The endeavour is therefore to keep the proportion thereof in the starting mixture as high as possible. Furthermore, by using a defined activated carbon, that is to say an activated carbon involving a specific pore size and a specific pore size distribution, it is already possible to implement pre-setting in relation to a subsequent area of use. In order however to be able to ensure economical processing and a stable end product, it is however necessary to add other auxiliary substances and additives. In that respect the man skilled in the art is familiar with the notion that activated carbon alone and in particular without the use of any binding agents and other additives is practically impossible to extrude and is also of inadequate stability in respect of shape. In accordance with the invention the proportion of activated carbon in the starting mixture is between 30 and 40% by weight. With a proportion thereof in the range of between 32 and 38% by weight of activated carbon in the mixture of the starting constituents, that overall affords a good adsorption capability with good product properties in terms for example of stability and electrical conductivity while with a proportion of between 34 and 37% by weight of activated carbon in the mixture of starting constituents, it is possible to obtain a product which at the present time is considered to be an optimum and which satisfies the demands made in respect of an adsorption filter, while at the same time the mixture of the starting constituents can be particularly well processed.
Water contained in the starting mixture is intended to ensure flowability and extrudability of the mixture of the starting constituents. It is generally present in the starting mixture in a proportion of between 23 and 34% by weight, preferably in a proportion of between 25 and 32% by weight and particularly preferably in a proportion of between 27 and 31% by weight.
The novolak which is used in accordance with the invention as a binding agent is a partially cross-linked phenol formaldehyde resin in powder form, which is of a melting point of between 80 and 160xc2x0 C., in particular between 100 and 140xc2x0 C. The novolak powder is contained in the starting mixture in a proportion of between 5 and 14% by weight, preferably in a proportion of between 8 and 14% by weight, more preferably in a proportion of between 10 and 130% by weight and particularly preferably in a proportion of between 11 and 12% by weight. Liquid phenol formaldehyde resins, referred to as resols, completely wet the activated carbon used and in part penetrate deeply into the pores which are present, and for that reason in accordance with the invention novolaks in powder form are employed. In that respect a particularly low degree of wetting of the activated carbon is achieved by virtue of using a novolak with a high degree of cross-linking.
In accordance with the present invention clay is present in the starting mixture in a proportion of between 5 and 50% by weight, preferably between 5 and 25% by weight and particularly preferably between 5 and 10% by weight and serves as a filler for hardening purposes while at the same time also serving as an extrusion additive or aid, by virtue of its flake or sheet-like structure. In that case it is possible to achieve a higher level of strength for the activated carbon molded body by virtue of an increase in the proportion of clay, which however results in a lower level of adsorption capacity of the activated carbon molded body, because of the comparatively smaller proportion of activated carbon. The added cellulose ether contributes to green stability and homogenises the mixture of activated carbon, novolak and clay, insofar as it prevents or at least sufficiently slows down separation of those constituents, by virtue of different densities. Cellulose ether is therefore contained in the starting mixture in a proportion of between 0.5 and 5% by weight, preferably in a proportion of between 1 and 4% by weight and particularly preferably in a proportion of between 2 and 3% by weight.
The liquid starch which in accordance with the invention is included in the starting mixture forms a stable lattice upon drying of the molded body produced from the starting mixture, and is thus of particular significance in regard to stability of shape of the green body. In accordance with the invention the liquid starch is contained in the starting mixture in a proportion of between 5 and 10% by weight, preferably in a proportion of between 6 and 9% by weight and particularly preferably in a proportion of between 7 and 8% by weight.
The wax contained in the starting mixture according to the invention assists with the shapinq or molding action and is contained in the starting mixture in a proportion of between 2 and 8% by weight, preferably in a proportion of between 3 and 6% by weight and particularly preferably in a proportion of between 4 and 5% by weight.
Polyacrylamide is also a binding agent which contributes to green stability and is contained in the starting mixture according to the invention in a proportion of between 0.2 and 1.2% by weight, preferably in a proportion of between 0.3 and 1./0% by weight and particularly preferably in a proportion of between 0.5 and 0.9% by weight.
Finally, in accordance with the invention the starting mixture contains soap which also serves as an extrusion additive or aid. It is contained in the starting mixture in a proportion of between 1.0 and 2.5% by weight, preferably in a proportion of between 1.2 and 2.0% by weight and particularly preferably in a proportion of between 1.5 and 1.7% by weight.
In a particular configuration of the present invention between 10 and 50% by weight of the proportion of soap in the starting mixture is replaced by graphite. Because, in contrast to the soap, the graphite does not penetrate into the pores of the activated carbon, this embodiment affords end products involving larger surface areas.
In another advantageous embodiment of the present invention, instead of activated carbon and novolak powder, the starting mixture contains a sulfonated polystyrene previously cross-linked with divinylbenzene.
The production of a sulfonation product of that nature is described for example in DE 197 52 593 and DE 199 30 732. The sulfonation products disclosed in those documents are preferably used in this embodiment, in place of activated carbon and novolak powder.
When applying the pyrolysis conditions set forth in the above-indicated documents in the state of the art, for the production of activated carbon from the sulfonation products, it is particularly advantageously possible to produce activated carbon molded bodies which can be matched to the corresponding situation of use in particular in terms of their pore structure and surface area.
For the purposes of further increasing the strength of the molded body, it is also possible to add between 1 and 15% by weight of glass fibers to the starting mixture, in relation to the overall mass of the above-described constituents of the mixture.
In production of the activated carbon molded body according to the invention, firstly the activated carbon is mixed with the novolak powder and then clay is added and further mixing is effected. In the alternative configuration in which the sulfonation product of a polystyrene cross-linked with divinylbenzene is used in place of activated carbon and novolak powder, the sulfonation product is mixed with clay. For that purpose, a mixture of water and cellulose ether is added and further mixing is effected, followed then by the addition of the remaining constituents with further mixing. In that respect the mixing operation is continued in each case to such an extent that the mixture is of adequate homogeneity at the respective moment in time.
After the operation of forming a molded body, in particular by extrusion of the starting mixture produced as described above, the molded body is dried. Many different processes are available to the man skilled in the art for that purpose, such as for example freeze drying or drying using infrared rays. It is preferable however if the molded body produced from the starting mixture is dried in a circulated-air oven at a temperature in the range of between 50 and 80xc2x0 C. or by means of microwaves. Those two processes ensure that rapid uniform drying takes place. When using microwave technology the drying time can be kept very short, in which respect it is readily possible to achieve a drying time of one minute without cracks or fissures occurring as a result thereof in the molded body or green product.
After the drying operation, the green body is subjected to pyrolysis which essentially serves for carbonisation of the novolak resin used. That is effected at a temperature in the range of between 650xc2x0 C. and 800xc2x0 C. under a nitrogen atmosphere. It will be appreciated that in that case the additional components such as wax, soap, the cellulose ether and the starch are also carbonised, in which case however there only remains from the starch and the cellulose ether a carbon lattice in the activated carbon molded body according to the invention.
Preferably, the temperature in the pyrolysis operation is increased until reaching the final temperature, at a heating rate of 5 K/min and the final temperature reached is then held for between 5 and 30 min. Subsequently to the pyrolysis operation, an activation operation can optionally also be effected, whereby additional activation of the carbonised novolak is possible and the active surface area can be increased by about 20%, in relation to the total filter.
Pyrolysis of that kind is also preferred if, as described hereinbefore, a sulfonation product is used instead of activated carbon and novolak in the starting mixture.
In principle the man skilled in the art is familiar with activation conditions for the activation of activated carbon and therefore activation is preferably to be implemented at a temperature of between 700 and 950xc2x0 C., with between 25 and 35% by volume of water vapor being meteredly added to the activation atmosphere.
An activated carbon molded body as described hereinbefore and in particular such a body in honeycomb form is preferably used as an adsorption filter, by virtue of its particular properties. In a particularly advantageous embodiment electrical contact elements are spaced from each other on the activated carbon molded body, in particular being arranged in mutually opposite relationship. It is possible to apply by way of those contact elements a current which, by virtue of the resistance of the activated molded body, results in heating thereof which, when a suitable air flow is supplied, resulted in desorption of previous adsorbed substances. In that situation, the procedure involves applying a voltage which is preferably switched off again after the maximum concentration of an adsorbed substance in the desorption air flow is exceeded. That makes it possible to effect rapid and very extensive desorption.